Cylindrical pressure vessels for holding one or multiple cylindrical cartridges useful in separation methods, such as crossflow filtration, are shown in a number of U.S. patents, including U.S. Pat. Nos. 4,717,035, 4,517,085, 4,781,830, 5,866,001, 5,720,411 and 6,558,544. Crossflow filtration is a type of separation using semipermeable membranes where only a portion of the feed liquid passes through the semipermeable membrane, with the remainder of the liquid flowing across the membrane, often along axially extending spirally disposed passageways, and exiting from the other end of the filtration cartridge as concentrate. Such cylindrical cartridges generally employ multiple, spirally wound, sheetlike membranes which may variously be reverse osmosis (RO), nanofiltration (NF) or ultrafiltration (UF) membranes. In such an arrangement, there has traditionally been an entrance or feed port near one of the pressure vessel ends and at least two exit ports from the pressure vessel, i.e. one at the other end for the remainder of the feed which is now a concentrated stream and one for the permeate (which may exit at either or both ends); these exit ports are often located in the end closures.
When multiple cylindrical cartridges of this type are included within a single pressure vessel, the liquid feed has generally entered one end, flowed serially through all of the cartridges and then exited at the opposite end of the pressure vessel as a retentate or concentrate stream; the permeate flows spirally inward to a central porous permeate tube in the cartridge. In such an arrangement, each cartridge will have an open, anti-telescoping plate (ATP) at each end, and some type of a connector will interconnect the permeate tubes of adjacent cartridges to create a combined permeate flow path centrally of the pressure vessel. Exemplary connectors 22 are shown in the '085 patent, and in order to save space between cartridges in a row within a pressure vessel, such couplings may be designed to reside substantially entirely within the permeate tubes, thus minimizing the distance between ATPs of adjacent cartridges. The permeate may exit at one or both ends of the pressure vessel.
All such connections in a pressure vessel which are to be subjected to superatmospheric pressure should of course contain suitable seals to prevent leakage. Generally O-ring seals, as shown at 49 in the '830 patent are used, but elastomeric seals of square cross-section, such as item 117 in the '411 patent, have also been used. Effective seals, e.g. chevron seals, are often also provided between the circumference of the cartridge and the interior cylindrical wall of the pressure vessel.
For some three decades now it has been found economical to use large arrays of pressure vessels, each holding multiple cylindrical filtration elements, which arrays are sized to provide the necessary amount of total membrane surface area to accomplish the desired rate of overall filtration/separation of a feed liquid stream. One example of such an array is shown in U.S. Pat. No. 3,880,755 (Osmonics, Inc.) which illustrates a number of groups of parallel pressure vessels supported in stacked arrangement and plumbed to create a plurality of banks of vessels, with each pressure vessel containing a plurality of spiral wound membrane modules or cartridges arranged for serial flow therethrough. These vessels were primarily made for dairy or other food processing uses, and the pressure vessels and the fittings were often made of stainless steel. As shown in FIGS. 1 and 2 of the patent, each of the pressure vessels in a horizontal bank was individually plumbed to a manifold so that each bank of 4 pressure vessels at a given horizontal level share feed from a common manifold. During the 1980's and continuing into the 1990's, Osmonics, Inc., and other manufacturers provided pressure vessels for operation in arrays where pairs of side inlet ports were provided at diametrically opposite locations at one end, through which each parallel vessel was linked to the next adjacent pressure vessel; thus, in such an array, connection of a single feed conduit to the outside vessel in a bank provided liquid feed to all of the interconnected vessels in the planar bank.
Since that time, U.S. Pat. No. 6,007,723 has issued, which describes the construction of various interconnectors which might be employed to link together adjacent pressure vessels of this general type in vertical stacks. Approximately contemporaneously, DHV Water of the Netherlands developed a system which used a different arrangement in a pressure vessel of a plurality of such cylindrical cartridges for filtration and/or separation operation, and presented a paper in 2003 describing their Optiflux® design. The proposed design feeds liquid from both ends of a pressure vessel of enlarged diameter with respect to the diameter of the cartridges, and utilizes connectors that interconnect both the central permeate flow tubes and the exterior cylindrical surfaces of adjacent cartridges. Exemplary interconnectors for the cylindrical, spiral wound membrane cartridges are described in U.S. Pat. No. 6,302,448. As a result, in a pressure vessel holding, for example, six cartridges, a pumped feed stream enters each end and flows serially through three cartridges; the two concentrate streams meet in the center at a special connector which interconnects only the permeate tubes. So-called radial “space holders” are provided which create an annular channel between the exterior surfaces of the linked cylindrical cartridges and the interior wall of the vessel. The concentrate streams enter this annular passage at the special center connector, and they are then discharged from both ends of the pressure vessel.
Although these above-described improvements in both pressure vessels for holding cylindrical cartridges and in arrays for carrying out filtration operations have shown some promise, the search has continued for even more efficient apparatus and methods for utilizing cylindrical filtration cartridges or elements, particularly those of the spiral wound membrane type.